Brown adipose tissue: a factor to consider in symmetrical tracer uptake in the neck and upper chest region.

Abstract

Increased symmetrical fluorine-18 fluorodeoxyglucose (FDG) uptake in the cervical and thoracic spine region is well known and has been attributed to muscular uptake. The purpose of this study was to re-evaluate this FDG uptake pattern by means of co-registered positron emission tomography (PET) and computed tomography (CT) imaging, which allowed exact localisation of this uptake. Between April and November 2001, 638 consecutive patients referred for PET/CT were imaged on an in-line PET/CT system (GEMS). This system combines an advanced GE PET scanner and a multirow-detector computer tomograph (Lightspeed, GEMS). The examination included PET with FDG and one CT acquisition with 80 mA. For CT, the following parameters were used: 140 kV, 80 mA, reconstructed slice thickness 5 mm, scan length 867 mm, AT 22.5 s. CT data were used for attenuation correction as well as image co-registration. Image analysis was performed on an Entegra work-station (ELGEMS). All patients with symmetrical uptake within the neck, thorax and shoulder regions were selected and the exact localisation of uptake determined (muscle, bone, fatty tissue or articulation). In 17 of the 638 patients (2.5%), increased, symmetrical FDG uptake in the shoulder region in a typical pattern was found. If extensive, this pattern included FDG activity comparable to brain activity in the lower cervical spine, the shoulder region and the upper thoracic spine in the costovertebral region. A less extensive pattern only involved intermediate FDG uptake in the lower cervical spine and shoulder region or in the shoulder region alone. In seven female patients (average 32.3 years), the extensive uptake pattern was seen. The average body mass index (BMI) was 19.0 (range 16.8-23.4). In the other ten patients (two male, eight female, average age 37.1 years), the average BMI was 22.7 (18.7-27.7). In all patients, the soft tissue uptake was clearly localised within the fatty tissue of the shoulders as demonstrated by PET/CT co-registration. The uptake in the region of the thoracic spine was localised in the region of the costovertebral joints. Symmetrical FDG uptake in the shoulder, neck and thoracic spine region is probably related to uptake in adipose tissue, especially in underweight patients. Hypothetically, this FDG uptake could represent activated brown adipose tissue during increased sympathetic nerve system (SNS) activity due to cold stress.